1. Field of the Invention
This invention relates to a process for preventing the scattering of glass fragments resulting from the bursting of a glass article such as a bottle by coating the surface of the glass article with a novel urethane resin composition. The process is also effective for protecting the surface of the glass article and for compensating for a reduction in strength ascribable to surface scratches or similar defects.
2. Description of the Prior Art
In order to increase the resistance of glass to bursting, a resin layer is usually coated on a glass surface. Such a resin coating has been applied to safety glass, glass bottles and photographic flash bulbs. Since the bursting of glass is accelerated by surface scratches on the glass article, the crystallization of the surface layer and abrupt changes in temperature, the coating of the glass surface with a resin is effective for preventing the strength of glass from decreasing.
However, coating materials for preventing scattering of glass fragments in the event of bursting of a glass bottle filled with carbonated beverages, beer or a similar gas-forming beverages require different properties from those used for reinforcing or surface-protecting of glass articles. According to the invention of our copending U.S. Pat. application Ser. No. 341,197 filed on Mar. 14, 1973 now abandoned, the coating resin for preventing the scattering of glass fragments caused by the bursting of a filled glass container such as a bottle (for brevity, hereinafter the description will be directed to the coating of bottles) must have an adequate tensile strength (on glass bursting), an adequate percent elongation (on glass bursting) and good adherence to the glass surface. All of these physical properties need not be superior at the same time, because their relationship, especially the relationship between the tensile strength and the percent elongation, is more important for obtaining favorable results.
More specifically, a very high tensile stength is not always required, and a suitable value can be determined in accordance with the value of the percent elongation. Thus, high tensile strength is required if the percent elongation is low, and even a relatively low tensile strength can be effective if the percent elongation is high. The lower limit of the tensile strength is about 50 kg/cm.sup.2 and that of the percent elongation, about 100%. The preferred ranges of the tensile strength and of the elongation are about 300 to about 600 kg/cm.sup.2 and about 200 to 600%, respectively.
An effective thickness of the resin coating is more than about 50 microns if the resin satisfies the above-described requirements, and preferably a thickness of from about 100 to about 200 microns. The optimum thickness is determined in accordance with the glass wall thickness and the inner pressure which the glass bottle to be coated will generally experience.
Methods of reinforcing a glass bottle by coating a film on the surface of a bottle are well known. For example, coating resins for glass bottle are disclosed in U.S. Pat. No. 3,178,049. This patent discloses a method for reinforcing a glass bottle. In contrast, the present invention relates to a method for preventing the scattering of glass fragments resulting from the bursting of a glass bottle by coating the surface of the bottle with a film of a novel urethane resin composition. Accordingly, this invention is quite different from the above-described patent. Furthermore, while a polyurethane resin is exemplified in U.S. Pat. No. 3,178,049 as a resin that can be used, no specific description appears with respect to the nature of the polyurethane resin used.
Many compositions are known for forming polyurethane resin films. For example, U.S. Pat. No. 3,428,609 discloses a process for manufacturing a solution in an organic solvent of a polyurethane. Some of the urethane resin films obtained from the solution possess the physical properties required of films. The composition disclosed in the above patent, however, contains an organic solvent, and it is generally known that compositions requiring an organic solvent are unfavorable since not only are they hazardous to the health of the working personnel but also cause pollution.
A solvent-free polyurethane resin composition is disclosed, for example, in U.S. Pat. No. 3,428,610. This patent describes a method for producing a cross-linked polyurethane, which comprises reacting an intermediate (urethane prepolymer) having a free NCO group with an aromatic diamine as a chain extending agent to form a highly elastic urethane resin. Although the composition thus prepared is suitable for casting, it is not suitable for coating. In addition, the curing after casting is effected by the action of moisture in the air, and from two to eight days are required for curing.
The invention described in our previous application includes a process for coating a glass bottle using solvent-free urethane resin compositions. However, these compositions are of a reactive two-package type having the defect of short storage period (or pot life).